Mailcow is single-host by design. Once you exceed it, three patterns work:
1. Vertical first. A 16 vCPU / 64 GB host handles ~2000 active mailboxes comfortably. Cheaper and simpler than splitting.
2. Split inbound vs storage. Front Mailcow with a dedicated MX relay (Postfix or HAProxy → Mailcow) to handle bursty inbound traffic. Mailcow still owns mailboxes.
3. Multiple Mailcow installs sharded by domain. Each domain lives on one host; a DNS-level MX pool routes per-domain. Simple, robust, doesn't need true clustering.
True HA? Mailcow doesn't ship clustering. DRBD-replicated vmail + MariaDB Galera + keepalived works but you own the integration. Most teams that "need HA" actually need faster recovery — invest in tested restore drills first.
🌍 Geo-redundancy: stand up a passive Mailcow in a second region, rsync vmail and mysqldump nightly. When the primary dies, switch DNS — recovery in <30 min.
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